Field of the Invention
The invention relates to a wire electric discharge machining device that intermittently generates an electric discharge in a machining gap formed between a wire electrode and a work piece to machine the work piece. In particular, the invention relates to a power supply device that repeatedly supplies current pulses to the machining gap through control of a switching element.
Description of Related Art
It is known that a capacitor-type power supply device generates an electric discharge in the machining gap by discharging a capacitor. The capacitor is connected in parallel to the machining gap. The capacitor-type power supply device is capable of supplying a current pulse that has a relatively steep rise. However, the supply of the current pulse depends on the charging and discharging of the capacitor. For this reason, the waveform of the current pulse is irregular and the surface roughness becomes non-uniform. In addition, for the capacitor-type power supply device, a relatively long off time is required in order to recover the insulation of the machining gap. Since there is a limit on the shortening of the rising time of the current pulse, it is difficult to increase the machining speed.
On the other hand, a non-capacitor-type power supply device does not include the capacitor and supplies the current pulse to the machining gap by controlling a switching element that is connected with the machining gap in series. Because the pulse width and the current peak of the current pulse can be easily controlled, the sizes of the crater-like holes formed by the current pulse are uniform. As a result, the surface roughness decreases, which is preferable.
In general wire electric discharge machining, multiple processes are carried out with different machining conditions in order to fabricate one product. First, a desired contour is roughly formed on the work piece. Then, in order that the dimensions of the contour meet the required values, the excess material is removed from the cut surface with high precision. Finally, finishing for improving the roughness of the cut surface is performed. As the processes come close to finishing, a current pulse of less energy is used. In the process before the finishing, most non-capacitor-type power supply devices supply a trapezoidal current pulse that has a constant pulse width to the machining gap. Such a current pulse improves the machining speed. However, since it cannot improve the surface roughness, the time required for finishing increases.
It is known that, if the current pulse is supplied to the machining gap at a high frequency of 1 MHz or more, the surface roughness is improved. Patent Literature 1 discloses supplying a current pulse to the machining gap at a high frequency by applying an AC voltage.